Devices of this type are used, for example, for receiving signals modulated by orthogonal frequency division multiplexing, commonly known as OFDM or COFDM signals.
Conventionally, digital signals, such as television programmes, are coded, in order to be broadcasted by radio relay channel.
Communication by radio relay channel generates propagation irregularities as a result of reflections and echoes caused by the environment, among other things.
It is thus possible to receive a plurality of data streams, corresponding to the reception of a single source signal, in a plurality of different ways, this phenomenon being commonly known as multiple reception.
Multiple reception may be a space diversity, by separating two receivers, so that the fading of the source signal is relatively decorrelated at these two points, but it may also be a polarisation diversity, a frequency diversity or a temporal diversity, or else a combination of these techniques.
When the various multiply received streams, all representative of a single source signal, are combined, a better-quality data stream may be obtained than if a single stream had been used.
The received streams may be combined by a plurality of methods. For example, the best stream may be retained, or else a threshold may be defined from which a stream may be used.
An effective method of combining multiply received streams consists in producing weighted additions.
A confidence index is thus attributed to each stream, which is processed as a function of said index, all of the processed streams then being added, in order to issue a total data stream.
The best combination or “maximum ratio combining”, commonly known as MRC, allows the maximum signal-to-noise ratio to be obtained at the output.
This combination is defined, in particular, in the article “Linear Diversity Combining Techniques” by D. G. Brennan, published in June 1959 by the ME (pp. 1075 to 1102).
The teaching of this article defines a combined datum or optimal ratio datum equal to the weighted data source for each channel.
The results obtained at the end of these combinations are sent to a weighted input decoder, such as a Viterbi decoder, for example, which decodes the total data stream in the conventional manner.
A device implementing a method of this type is also disclosed in the patent FR-B-2 788 048.
In this device, each reception channel issues a confidence index from this channel and data which is more or less amplified, as a function of this index.
All of the data and the confidence indices are used in a single combination step, issuing a total confidence index and a total data stream that are adapted to a weighted input decoder.
It thus appears in this device that the architecture is based on the juxtaposition of processing channels and on the use of a final, multiple input combination stage.
As a result, numerous functions are duplicated on each channel, and the summation/weighting function has to be incorporated on each channel or be implemented by a single, specially dimensioned circuit, which requires a considerable surface area for the substrate during production, in the form of electronic components.
It appears, therefore, that the existing devices are large, complex and expensive.